There exists on the market a multiplicity of prosthesis systems for replacing the natural hip joint in the event of painful, traumatic, arthritic or other changes. Usually, so-called modular systems are used wherein a socket insert is inserted into a hip socket consisting in most cases of a metal alloy, which socket insert forms a part of the artificial sliding bearing and which can consist of a metal alloy, a ceramic material, a plastic or a composite of the aforementioned materials. Coupling between the socket insert and the hip socket often takes place through a so-called conical clamping, wherein a conically shaped portion of the outer geometry of the socket insert together with a matching conically shaped portion of the inner geometry of the hip socket forms a frictional connection, see FIG. 1.
One of the problems which may occur intra-operatively is the tilted insertion of the socket insert into the hip socket. Then, instead of the described conical clamping, jamming of the socket insert can occur between three contact points within the clamping cone of the hip socket. Depending on the magnitude of the force applied during clamping, such high frictional forces occur as a result of the punctual jamming that the position of the socket insert can no longer be corrected intra-operatively, see FIG. 2.
The consequences for the function of the hip joint prosthesis in the case of a socket insert that is inserted in a tilted manner substantially depend on the material of the socket insert and range from increased wear to corrosion and to complete destruction of the socket insert. Thus, a socket insert inserted in a tilted manner can be the cause for a subsequent, complicated and expensive revision surgery.
In order to avoid the tilted insertion of socket insert, a number of insertion instruments are available on the market. Their function is substantially based on the following three steps:                1. Gripping the socket insert at the upper outer edge.        2. Aligning the instrument including the socket insert relative to the hip socket so that the axes of symmetry of the hip socket and the socket insert run in parallel,        3. Abruptly and rapidly pushing the socket insert into the hip socket while releasing the grip and establishing the clamping connection.        
EP 1 076 537 B1 and DE-U-299 22 792 describe insertion instruments wherein the socket insert is gripped at the upper edge by three holding claws. Furthermore, these insertion instruments provide a short handle piece at which the surgeon can grab the insertion instrument and can insert it together with the socket insert into the operating field. There, placing it onto the hip socket and finally joining the socket insert takes place. The disadvantages of these solutions—little feedback through palpation for the surgeon, required space for the instrument makes inserting and packaging difficult—are resolved by an insertion instrument according to DE 10 2009 054633.
Direct manual handling during which the hand is moved directly into the operating field and possibly touches the wound tissue is viewed by different surgeons as being disadvantageous. From the surgeons' view, this can result in an increased risk of infection. Furthermore, the visibility of the operating field is temporarily extremely limited. For the mentioned reasons, some surgeons prefer to carry out the handling of prosthesis components through instruments by means of a long handle and a handhold at the end.
WO 2008/106598 describes an insertion instrument wherein a kind of an elastic cover is pressed onto the socket insert. The edge of the cover engages with the socket insert over the entire circumference. Through an opening in the cover including a radial slot, an impacting instrument or the combination of socket insert and cover is attached onto an impacting instrument. By means of the impacting instrument, the socket insert is introduced into the operating field and joined.
The disadvantages of the described solution are:
                Due to the radial slot in the cover opening, the coupling between cover and impacting instrument is slightly flexible; however, it does not permit any major tilting of the socket insert with the cover relative to the axis of the impacting instrument. This means, when introducing the socket insert into the operating field, the socket insert is aligned substantially perpendicular to the axis of the instrument handle. Thus, aligning the socket insert and the hip socket does not take place automatically, but has to be actively performed through the surgeon.        The edge of the cover encloses the entire circumference of the socket insert, and for a correct alignment of the socket insert relative to the hip socket, has also to rest over the entire circumference on the socket end face. If this is not possible due to protruding tissue rests, e.g., osteophytes, no correct alignment of the socket insert prior to impacting is possible. As a result, the risk of tilted insertion increases, or the instrument cannot be used.        Due to the complete enclosure of the socket insert by the cover, the view on the end face of the of the hip socket when inserting the socket insert is obstructed so that the correct alignment of the socket insert relative to the hip socket cannot be controlled visually.        